Always beware the beguiling narcissism of small differences. The contrast in performance between a national exam system in which 65.4% of entrants gain the top A*-C grades one year, and one in which 63.9% gain them the next, is not, on the face of it, very great at all. Year on year, the scores are more or less the same. But when the change is also the first dip in 26 years in the otherwise steady upward trajectory of GCSE English scores, when it also follows a series of changes in the English exam designed to make it tougher, and when it comes after new efforts by the exam regulator to put the squeeze on grade inflation, this is a moment worth taking seriously.

True, English is only one paper among the dozens of different GCSE courses where exams were sat this summer, and whose results were published on Thursday. But the downturn in top success rates in English, while the most controversial example for many in many English schools (the new exam has not been approved in Wales), is also representative of a wider picture. Taking all this year's GCSE results together, there has been a fall in the proportion achieving the top A* grade of 0.5% compared with last year, while the proportion achieving A*-C grades is also down by 0.4%. As in the case of English, this is the first fall since the GCSE qualification was introduced in 1986. And it follows a similar fall of 0.4% last week in the top grades at A-level this year.

Clearly, therefore, this is a pattern. Not a spectacular change, maybe, but a change nevertheless. Equally clearly, the change is deliberate and has been fostered from the centre, both by the education secretary Michael Gove himself, and indirectly through pressure on the exam boards. Mr Gove denied a directing role on Thursday, implying that the changes are the work of the boards. But it will be interesting to see whether this ambitious minister is so bashful when he comes to address the Conservative party conference this autumn.

In fact, there is not necessarily anything wrong in principle with an effort, whether directed from the centre or anywhere else, to ensure that grades are not creeping upwards unjustifiably. Yes, the issue of grade inflation has undoubtedly long been a shibboleth on the right – especially among those who actively despise state schools. And, yes, the issue is umbilically, and certainly not always fruitfully, linked to the target-culture approach to education reform.

But it is also important that the public – and this means students, colleges, parents and employers too – can retain absolute confidence in the currency of exam results. In most cases that confidence is still well placed, partly because standards really have been lifted over the years, by teachers above all. But there is some evidence that grade inflation has been allowed to creep into some of the process, not least as a result of competition between boards. And there is also real evidence of too many students entering the labour market with inadequate English and other basic qualifications too. So it is equally clearly the responsibility of regulators, boards and ministers to bear down on these things.

What won't do, however, is to squeeze the system remorselessly, either as an end in itself or as a means to a separate purpose. Regular maintenance of the exam system is one thing. But repeatedly denigrating, or appearing to denigrate the system, as so many on the right prefer to do, is both destructive to schools and deeply unfair on the students whose futures risk being sacrificed in the process.

It would be wrong if this grade squeeze was used as a lever to force schools prematurely into closure or enforced academy status. Such outcomes are certainly not objectionable in all circumstances. Children and parents are entitled to high expectations and confidence about schools. But radical change should wait on genuine evidence of chronic underperformance in each case. It should not be meted out as punishment for a one-off, marginal or centrally contrived failure to meet what is ultimately another arbitrarily changed target.